A video display described in U.S. Pat. Nos. 6,181,367 and 5,973,727 to McGrew et al., both of which are hereby incorporated by reference, uses a liquid crystal layer to switch totally internally reflected (TIR) light out of a plate at individual display rows. Activating strip electrodes alter the refractive index of the liquid crystal layer along individual display rows in a timed sequence with an image generating display signal injected endwise into the plate. A separate optical element focuses the switched-out light onto the pupil of the user's eye.
This approach has some shortcomings. First, the vertical eyebox is quite limited. Second, troublesome vertical diffraction of the switched-out image light can occur through narrow individual display rows. An object of the present invention is to solve these problems by using a traveling lens to direct light from multiple rows in the display into a vertically collimated beam directed toward the user's eye. A beam steerer may be considered to be a special case of a traveling lens, in which the traveling lens has an infinite focal length and a variable-direction axis.
Any device that can actively redirect a beam of light can be considered a beam steerer. An ordinary diffractive or refractive lens redirects light passively, with the angle of redirection varying continuously across the lens. A variable-focus lens, though, could be considered a crude type of beam steerer because the angle of redirection at any point would be actively controlled as the focus is varied. A movable lens, movable prism, or deformable mirror could similarly be considered to be a beam steerer. Another implementation of an active optical element to serve as a beam steerer or traveling lens would be an acoustically or otherwise generated diffractive optical element.
Several different physical phenomena can be used to implement a traveling lens or beam steerer in this display. The electric field across a liquid crystal layer can control the effective refractive index of the liquid crystal, so spatially varying electric field produces a spatially varying optical thickness that can function as a lens or as a prism. An acoustic wave train in a solid or liquid can act as a diffraction grating (effectively a prism) or as a diffractive lens depending on the shape and frequency of the wavefronts. An acousto-optic diffraction grating results from an acoustic wave of constant frequency, while an acousto-optic lens results from an acoustic wave with a frequency that varies approximately quadratically with respect to time. The refractive index of a photo-refractive material depends on the intensity of light impinging on the material, so a photorefractive lens can be generated by illuminating a plate of photorefractive material with a light beam whose intensity varies approximately quadratically with respect to position on the plate. An acoustic wave in a liquid crystal material affects the refractive index of the material, so a prism or lens can be created in liquid crystal layer by passing an acoustic wave through the layer with appropriately controlled frequency “chirp”: a prism would have no chirp, whereas a lens would have a quadratic chip. A high-intensity acousto-optic wave in a medium like water or silica can act like a traveling array of lenses, where each wave cycle acts like a single lenslet.
According to the present invention, the vertical eyebox of the display that is the subject of U.S. Pat. Nos. 6,181,367 and 5,973,727 can be widened by generating a traveling lens that moves vertically across the display. The traveling lens preferably directs light (corresponding to one row of image pixels) out from a relatively wide band of display rows as a quasi-collimated beam approaching a user's eye to form a virtual image of the row of image pixels (preferably at infinity). The vertical eyebox could then be as wide as the band of display rows. In this case, light coupled out of a TIR (total internal reflection) mode in the plate can be directed into any direction desired, then redirected by the traveling lens. For example, light can be coupled out of the TIR plate by a low-efficiency linear diffraction grating into a collimated beam (for each LED light source) propagating perpendicular (with respect to the vertical direction) to the plate's surface; then a beam steerer can direct the collimated beam to the eyebox. Alternatively, light can be coupled out of the TIR plate by a diffractive element such as an HOE (holographic optical element) or an acoustic wave pattern, to focus to a line at the eyebox, and a traveling lens can collimate a portion of the light into the eyebox.
Also according to the present invention, it is possible to eliminate unwanted diffraction from liquid crystal electrodes and from narrow shutter slits by use of the traveling lens. If the traveling lens has effectively infinite focal length, it is essentially a variable prism. In that case, the width of the band of display rows used to display each image row instantaneously can be the full height of the display. The entire display would emit light into one vertical angle as a collimated beam, and the angle would sweep approximately 30 times per second through its full range. As the angle sweeps, the power of the LEDs producing the light beam is modulated synchronously with the sweep so that the image information in the beam corresponds at each moment to the vertical angle of the redirected beam. An eye in the eyebox will perceive a two-dimensional image. If a different image from a stereo pair is presented to each eye, a 3D image can be perceived.
The traveling lens can be implemented in many different ways. One way is to place a layer of liquid crystal between two arrays of electrodes. For any one polarization axis, the refractive index depends on the electric field in the liquid crystal layer. For convenience, the direction perpendicular to the layer of liquid crystal can be called the Z direction, the vertical direction can be called the Y direction, and the horizontal direction can be called the X direction. When voltages are applied to the electrodes to create an electric field in the Z direction that varies approximately quadratically with X, Y, or both, the liquid crystal layer will act as a lens. If the coefficient of the quadratic term goes to zero and the coefficient of the linear term stays finite so that the focal length of the lens goes to zero but the refractive index varies linearly with respect to X or Y (or both) the liquid crystal layer will act as a prism, with the effective wedge angle of the prism depending on the coefficient of the linear term.
If the wedge angle of a prism varies with time, a light beam passing through the prism will be deflected by an angle depending on the wedge angle. So, the liquid crystal device can act as a beam steerer.
If the focal length of a lens varies with time, it can serve as an adaptive lens. If the center of its curvature moves in the X or Y direction (or both) but the focal length stays constant, the lens is a simple traveling lens. This may be accomplished by varying voltages in the liquid crystal device as follows: let the voltage across the liquid crystal layer be V, such that V=Vo+a′(X−x′)+b′(Y+y′)+c′(X−x′)2+d′(X−x′)(Y−y′)2+e′(Y−y′)2. Now vary x′ or y′ (or both) as a function of time so that x′=x′(t) and y′=y′(t). In effect this moves the lens axis to new positions (x′,y′)=(x′(t),y′(t)). If x′ and y′ vary linearly with time, the lens will move at a constant velocity.
Of course, because liquid crystals respond nonlinearly to an applied electric field, the ideal voltage profile also will be nonlinear. Similarly, the dynamic response of a liquid crystal layer depends on various factors including the rate of change of the applied voltage, the composition of the liquid crystal material, the temperature of the liquid crystal, and the impedance, capacitance and geometry of the electrodes. Accordingly, in order to generate a high quality liquid crystal traveling lens that moves fast or a liquid crystal prism that can deflect a light beam rapidly without distortion, it is necessary to take the nonlinearities and the dynamics into account and pre-compensate for the expected distortion.
Some researchers have developed “modal” variable focal length liquid crystal lenses (see reference immediately below), in which an AC radio-frequency voltage produces a DC lens. Typically, these lenses are generated by a high resistivity disk-shaped electrode in electrical contact with a surrounding low-resistivity electrode. Varying the frequency and amplitude of the applied AC voltage results in a changing focal length lens. It is possible to accomplish a very similar effect with an AC-driven electrode array. Details of such lenses are discussed in a paper entitled “Adaptive modally addressed liquid crystal lenses” by Philip J. W. Hands, Andrew K. Kirby, and Gordon D. Love in Liquid Crystals VIII. Edited by Khoo, Iam-Choon. Proceedings of the SPIE, Volume 5518, pp. 136-143 (2004), which is hereby incorporated by reference. Other adaptive optics useful for purposes of this invention are discussed in the following papers: “Liquid-crystal adaptive lenses with modal control,” by A. F. Naumov, M. Y. Loktev, I. R. Guralnik, and G. Vdovin, in Optics Letters 23, 992-994 (1998); “Control optimization of spherical modal liquid crystal lenses” by A. Naumov, Gordon Love, M. Yu. Loktev, and F. Vladimirov in Optics Express, Vol. 4, Issue 9, pp. 344-352; and “Wave front control systems based on modal liquid crystal lenses” by Loktev, M. Yu.; Belopukhov, V. N.; Vladimirov, F. L.; Vdovin, G. V.; Love, G. D.; Naumov, A. F. Review of Scientific Instruments, Volume 71, Issue 9, pp. 3290-3297 (2000), all of which are incorporated by reference.
One method for optimizing the shape of the driving signal is to use a genetic algorithm. For example, the shape of the driving signal could be controlled by a set of variable resistors in an RC or RLC circuit. The genetic algorithm could treat the values of the resistors as “genes” in a “chromosome”, and treat the measured performance of the traveling lens or beam steerer as the “fitness”, to find an optimum set of resistor values via Darwinian evolution. The use of genetic algorithms for optimizing multiple—parameter systems is well known.
Another way to implement the traveling lens or beam steerer is as an acoustic diffractive element. Previous work with acoustic traveling lenses has been documented by R. H. Johnson and R. M. Montgomery in a paper entitled “Optical beam deflection using acoustic-traveling wave technology” published in Proc. SPIE, Acousto-Optics/Instrumentation/Applications, Vol. 90, p. 43, Aug. 1976. The paper describes a traveling acoustic wave device for use as a document scanner, in which a “chirped” acoustic signal serves as a diffractive lens.
L. Onural, G. Bozdagi, and Abdulla Atalar describe an electronically generated instantaneous hologram in an article entitled “New high-resolution display device for holographic three dimensional video: principles and simulations” in Optical Engineering/March 1994 Vol. 33 No. 3/p. 835. The hologram is in the form of a pattern of surface acoustic waves, which diffracts light to form an image that can be three-dimensional and can contain both horizontal and vertical parallax.
A related device is described by Smalley, Bove, and Smithwick at MIT Media Lab in the MTL Annual Research report September 2006. The device uses guided acoustic waves interacting with total internally reflected light to create a 3D instantaneous hologram having only horizontal parallax. The acoustic waves diffract light out of the total internally reflected mode and into air to form the image.
Other examples of acoustic traveling wave lenses along with their control systems are disclosed in U.S. Pat. Nos. 6,043,924; 6,052,215; and 6,538,690, to Montgomery et al., which are all hereby incorporated by reference.
It is known that an ultrasonic acoustic wave can alter the optical properties of a liquid crystal layer, so it is possible to form an optically diffractive structure by propagating high-frequency acoustic waves through a liquid crystal layer. The frequency of the acoustic waves would have to be selected so that the wavelength of the acoustic waves in the liquid crystal medium (or at the interface between the liquid crystal layer and an adjacent medium) is comparable to the wavelength of the light that is to be controlled by the device.